This invention relates to television receiver tuners and more particularly to such tuners that are used to detect and process both digital and analog signals.
Conventional tuners are composed of discrete devices, such as tuning coils, tracking filters and phase locked loop filters, which require manual tuning. These external tuning requirements result in extra expense and add time to the manufacturing process. Generally, prior art tuners are designed to process a narrow range of frequencies at any one time. This is accomplished by a tracking filter on the front end of the tuner. As the receiver is tuned across the frequency band during a channel change the tracking filter is tuned to allow only a few channels to pass into the tuner. As a result, the tuner circuit only has to work with a few channels instead of the entire bandwidth. For example, in a cable television system the tuner would allow only a few channels to enter the receiver front end, instead of the full 100 or more channels in the total cable set. The cable channels could be at full strength of about 15 dBmV. The effect of the tracking filter is to reduce the dynamic range required in the front end of a conventional receiver.
For analog television signals there is a large picture carrier which contains most of the power, this requires a large receiver dynamic range. The analog television signal waveform is vestigial side band (VSB), which is similar to single side band transmission. However, in VSB transmissions there is some signal spillage over to the unwanted side band. To compensate for this unwanted spillover, a special filter is used in the intermediate frequency (IF) processing portion of analog tuners to suppress the carrier approximately 6 dB to compensate for the difference in side band signal level. Typically, surface acoustic wave (SAW) filters are used to suppress the unwanted side band. It is critical that the carrier signal be positioned properly on the frequency response of the SAW filter to ensure that the picture carrier passes.
Analog television signals contain an audio carrier and a chroma carrier in addition to the main picture carrier signal. The audio carrier frequency is 4.5 MHz higher than the picture carrier. Normally there is a filter, such as the SAW filter in the IF, which suppresses the audio carrier approximately 20 dB to prevent intermodulation problems between the carriers in the video channel. The picture carrier, audio carrier and chroma carrier tend to beat together and cause spurious signals.
Digital television signals can be used instead to overcome the problems caused by the format of analog television signals. Although the format for digital television signals has not been standardized to date, there are common pieces among the proposed formats. Primarily, the digital signal is expected to have band shape that is fairly flat across approximately 6 MHz. This 6 MHz bandwidth could have within it several different selectable regions (channels) of intelligent signals. The digital TV reference signal will be small and would be about 6 dB below the average pictures (or desired data) signal level. The digital signal would have information spread uniformly across the 6 MHz band. Sound would be part of the signal and there would not be a separate digital sound carrier. Thus, the tuner must be functional to capture each 6 MHz band and then allow for decoding therefrom one or more channels of programming or data.
Whichever final standards are chosen for digital television, there will be significant variations between the analog and digital signal formats. Therefore, existing processors which currently handle analog signals will not be able to process future digital signals. An example of a television tuner having no front end tracking filter is disclosed in the above-referenced co-pending application entitled MONOLITHIC TELEVISION TUNER. However, the television tuner disclosed in that application for patent does not provide for tuning television signals in more than one format. Therefore, in order to receive both analog format signals and digital format signals, a television would have to use two or more of such tuners, where each tuner is designed to receive a different format. If a television used multiple tuners, it would also need some way of determining when to select between the various tuners in order to properly receive different channels that each have a different format.
The present invention allows both analog and digital signals to be received by a single tuning circuit which may, if desired, be embodied as a single integrated device. The narrow band tracking filter of the prior art is replaced by a front end filter having a wide band pass that permits all channels in a desired band to pass into the circuit. Frequencies above the desired band are rejected.
A dual conversion circuit is used to convert a desired channel in the received signal to an intermediate frequency for further processing. The first mixer of the conversion circuit has a high dynamic range which allows it to receive all channels, voltage controlled oscillators (VCOs) driven by multiple phase locked loops (PLLs) are used to provide the local oscillator (LO) signal for the conversion circuit mixers. The PLLs allow for precisely stepped LO frequencies in the conversion circuit.
A first intermediate frequency filter (FIFF) operates in conjunction with the second conversion circuit mixer to provide image rejection. Two second intermediate frequency filters (SIFFs) are selectively switched into the intermediate frequency (IF) signal path to create an overall bandpass characteristic that is alternatively suitable for either digital or analog television signals. An automatic carrier detection (ACD) circuit monitors the output of both SIFFs and determines whether the signal being processed is in the digital or analog format. The output of the selected SIFF combination is an IF signal that is provided either to an off-chip decoder device or to additional on-chip circuitry for further processing.
A coherent oscillator (COHO) circuit is used to create both in-phase (0xc2x0 phase shift) and quadrature (90xc2x0 phase shift) reference signals from the IF signal. A frequency discriminator monitors the output of the COHO to ensure proper signal tracking.
A first mixer receives the in-phase reference signal and operates as either a video detector for analog signals or an in-phase detector for digital signals. The video detector provides signals to both composite video and digital I-channel circuits for further processing. A third signal from the video detector is provided to an automatic gain control (AGC) circuit. The AGC circuit controls the overall gain by adjusting the gain of an IF amplifier at the output of the SIFFs and a delayed amplifier at the front end of the tuner.
A second mixer receives the quadrature reference signal from the COHO and operates as either an audio down converter for analog signals or a quadrature detector for digital signals. The audio down converter provides signals to an audio detector and to a digital Q-channel circuit.
The overall operation of the circuit is such that a broadcast signal is received through the front end filter and then converted to an IF signal by the dual conversion circuit. The IF signal then passes through the SIFFs while the ACD circuit monitors the outputs of the SIFFs to determine whether the signal is in analog or digital format. Once the signal type is determined, the second SIFF is switched in or out of the IF signal path as appropriate to provide the proper overall bandpass characteristic for that signal type. The output of the SIFFs can be provided to an off-chip decoder device or it may remain on-chip. For digital signals, after passing through the in-phase and quadrature detectors the signals are further processed and output as the digital I and Q channels. For analog signals, after passing the video detector and audio down converter, the signals are processed and output as composite video and audio signals. The overall control of the circuit is accomplished through a control interface circuit and a processor, such as a computer.
It is one technical advantage of the present invention to provide a tuner circuit having a front end with a high dynamic range which allows reception and processing of all channels in a desired band.
It is another technical advantage of the present invention to provide a tuner that is capable of determining whether a received channel contains an analog signal or a digital signal and after identifying the signal type automatically adjusting and optimizing the overall frequency response of the tuner.
It is a further technical advantage of the present invention to provide a tuner which is capable of processing analog and digital signals on a single integrated device.
It is a further technical advantage of the present invention to provide a tuner that can be used in a variety of applications beyond a conventional television set. The present invention can also be incorporated into a computer or other device as an integral component or as an add-in board.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.